Researchers at the University of Exeter say it's all because of the body's fight-or-flight response. The stress response that was meant to keep us safe in the wild millions of years ago gets tripped in our modern life, too.

In the study, published in the journal Clinical Psychological Science, 135 healthy students were took part in an experiment in which one group was asked to listen to audio instructions that stressed positivity and self-encouraging thoughts. The other group listened to a more critical voice.

The groups' vitals were taken, and those in the latter group reported elevated stress levels, measured by heightened heart rates, sweat, and other markers.

The researchers explained that positive thoughts kept the flight-or-fight response at bay, and that actually boosts the body's immune response.

Karl added, "We hope future research can use our method to investigate this in people with mental health problems such as recurrent depression."